If you created the image with the type tool then yes you can edit the type. If you purchased the image do a Command Y to go into outline mode, if you see the type filled with black you can edit, if outlined then it is paths and not editable. Do a Command Y to go back to preview mode.
If the type is not editable you will need to recreate with the type tool.
If the type is editable you can double click with the pointer tool, and your tool will change to a bar for editing type.